A metallized plastic film capacitor (hereinafter referred to as "MF capacitor") made by rolling up a metal deposited plastic film which has an electrode of vacuum-deposited metal layer such as an aluminum layer on a plastic film, has a self-healing function and a high dielectric strength. The capacitor of this kind is suitable for reducing the sizes, and accordingly, it is widely used.
As plastic films used for MF capacitors, biaxially stretched polypropylene films are widely employed because they are inexpensive and excellent in temperature characteristics of dielectric loss.
In the production of foil-rolled capacitor which is made by rolling up a plastic film such as polypropylene film together with a metal foil of aluminum or the like, or a single- or double-side metallized paper as an electrode, the impregnation with an electrically insulating oil is commonly employed. For example, Japanese Laid-Open Patent Publication No. 60-35408 discloses that an insulating oil consisting of 20-50% by volume of diarylethane, alkylnaphthalene or the like and the balance of rapeseed oil, is used for the impregnation into a capacitor that is made by rolling up a polypropylene film.
In the foil-rolled capacitors of this kind, only the impregnating properties of insulating oils relative to plastic films should be taken into consideration.
In the case of the MF capacitor, however, even when small dimensional change of base film is caused to occur by an insulating oil, or a small amount of impregnating oil penetrates into a space between a metal deposition 10 layer and a base film, cracks occur in the metal deposition layer, and, in an extreme case, the metal layer is peeled off form the base film to cause dielectric breakdown. These phenomena such as the dimensional change of base film and the penetration of impregnating oil between a metal deposition layer and a base film, occur mainly due to the penetration of the impregnating oil into the base film.
The thickness of a metal deposition layer is very small and the metal is liable to evaporate when it absorbs the energy of partial discharge. Although this effect implies the above-mentioned self-healing function, it also causes several drawbacks such as cracks and peeling in a metal deposition layer, which is not desirable because it is liable to cause partial discharge at a lower value in potential gradient.
In view of the above facts, the aromatic hydrocarbons such as diarylethane and alkylnaphthalene as disclosed in the above-mentioned Japanese Laid-Open Patent Publication No. 60-35408 are not suitable for use in the impregnation of MF capacitors, because when they are impregnated into a polypropylene film, they exhibit a high degree of swelling and cause the peeling of metal deposition layer as described in the above patent gazette. It is, therefore, impossible to improve sufficiently the above-mentioned aromatic hydrocarbons which seriously swell the polypropylene film by impregnation.
Incidentally, when insulating paper is used as a base film of metal deposition film, the trouble such as the peeling of a metal deposition layer due to the penetration of an insulating oil, is hardly caused to occur, because the paper is a complex material made of pulp fibers and is polar to some extent. In other words, the above problem such as the peeling of metal deposition layer is characteristic of metal deposited plastic films.
When an MF capacitor is impregnated with an insulating oil, it is impossible to use an excessively viscous impregnating oil because the impregnating operation into capacitor elements is hardly carried out. In general, however, an impregnating oil having a higher viscosity hardly penetrates into a base film. As a result, the dimensional change of base film and the penetration of an impregnating oil between a metal deposition layer and a base film can be suppressed. Therefore, an impregnating oil having a higher viscosity is not always unsuitable for the impregnation of MF capacitors. In view of this fact, the description in the above-mentioned Japanese Laid-Open Patent Publication No. 60-35408 that the castor oil is not suitable because of its high viscosity cannot be applied to the case of MF capacitors.
Besides the above-mentioned Japanese Laid-Open Patent Publication No. 60-35408, Japanese Laid-Open Patent Publication No. 61-45510 (U.S. Pat. No. 4,591,948) discloses an MF capacitor which is impregnated with 1-phenyl-1-(benzylphenyl)ethane and it describes further that animal and vegetable oils such as castor oil can be used in a mixture.
According to the experiments carried out by the present inventors, high corona starting voltages and also high withstand voltages were obtained when MF capacitors were impregnated with the aromatic hydrocarbons disclosed in the above-mentioned Japanese Laid-Open Patent Publication No. 61-45510. In spite of the high withstand voltage, however, the durability was not always good in that it was several ten hours under constant voltage application at a potential gradient of 130 V/.mu.m. Although the corona starting voltage is high, the durability under the charge of constant voltage is not so high. Because the durability under constant voltage application is rather important in practice, the improvement in this regard is desired.
Furthermore, according to the experiments carried out by the present inventors, when MF capacitors were impregnated with an animal or vegetable oil such as castor oil or rapeseed oil, the results were different from those with the above-mentioned aromatic hydrocarbons. That is, both the durability under constant voltage application and the corona starting voltage were low.
In view of the above circumstances, the present invention proposes a practically useful oil impregnated MF capacitor which has excellent durability under the charge of constant voltage.